Can't figure out how to fix non-manifold geometry in my game title
You sure you merged by distance? I tried it upon first opening your file and it removed 195 vertices right from the get go (shown at bottom).

As you can see from the face orientation overlay, half of your mesh has faces facing inwards and the other half outwards. There is also a problem with the circled face (there may be a couple more instances). It looks connected, but it is in fact overlapping (you can tell by the z-fighting). This may be a result of the merge, but it needs attention nonetheless.

You can fix the normal direction by selecting all and then selecting mesh &gt; normals &gt; recalculate outside, but the face will have to be edited and &quot;re-connected&quot; manually.